The Measured Ecological Impact of Grass Carp

Despite the use of Grass Carp as an effective control strategy for nuisance aquatic macrophytes, there has been uncertainty about its perceived risk to North American aquatic ecosystems. Recent observations of feral (and sometimes diploid) individuals in the Great Lakes (GL) have spurred interest in re-evaluating the mechanisms by which Grass Carp interact with ecosystems. Here we assess the ecological impact of Grass Carp using a suite of tools including expert questionnaire, ploidy assessment, eDNA surveillance, field observations including diet evaluation, species distribution models (SDM) and meta-analysis. While expert perceptions of whether Grass Carp may impact GL ecosystems were variable and in some cases, uncertain, we found that some ecosystems may be more vulnerable than others. Feral, and sometimes diploid, individuals occurred in GL waterways and eDNA surveillance suggests that Grass Carp are abundant in tributaries. SDMs predicted that Grass Carp establishment in likely in all 5 of the Great Lakes and habitat suitability was correlated with observed growth rates of wild populations. Meta-analysis results showed that Grass Carp impact both biotic and abiotic components of ecosystems. These findings indicate the potential for ecological impacts by Grass Carp and that iterative risk assessment of introduced fishes may be warranted.